Wavelength-division multiplexing (WDM) has been employed in core networks for almost a decade. WDM technology enables signals of multiple wavelengths to be concurrently transmitted over a given optical medium. This has been facilitated by the availability of wideband optical amplifiers that can simultaneously amplify many different wavelengths without distortion. The advantages provided by WDM translate into greater fiber utilization, lower capital expenditures associated with fiber deployment, and reduced costs in repeater stations by eliminating the need to terminate each wavelength along the fiber path. To maximize economic utility, the wavelength density that can be multiplexed onto a given fiber has increased in recent years: 80-wavelength systems are now common in the EDFA band, with 50 GHz frequency spacings between channels in many offerings. However, this dense WDM (DWDM) technology has not penetrated into local applications for several reasons, primarily including the relatively high costs per circuit arising from the fixed cost of equipment in local applications where the traffic volumes per route are relatively low. Recent developments in low-cost coarse WDM (CWDM) components have made this technology more practical for local applications. CWDM channel spacings are wider than DWDM spacings by a factor of about 50 or so. This relaxed tolerance translates into much easier fabrication, processing, and qualification procedures. An example of a CWDM application is described in co-owned Frigo et al. (“Frigo”) U.S. patent application Ser. No. 10/127,195 entitled VIRTUAL COMMON SPACE: USING WDM IN METRO ACCESS NETWORKS, filed Apr. 22, 2002, the disclosure of which is incorporated by reference herein. The Frigo disclosure relates to a technique for serving individual high-volume fiber-to-the-floor (FTTF) customers in a building by installing low-cost CWDM multiplexers and demultiplexers in the basement. These devices are passive optics and capable of being installed inside wall-mounted splice enclosures, thus avoiding the costs associated with installing traditional high-speed multiplexers in the common space of the building. Other applications for CWDM include reinforcing existing metro rings that connect inter-office facility local network services (LNS) offices.
In WDM access applications, a plurality of optical channels, each to a different customer, are multiplexed onto an optical fiber. In an exemplary installation, riser fiber is installed between the common area of a building and customer premises in the building. The riser fibers are coupled to a demultiplexer that receives multiplexed signals from trunk fiber in the street, and a multiplexer for multiplexing signals from the customer site back to the trunk. These passive WDM nodes establish a connection with each individual customer over an assigned wavelength band. Craft personnel have to splice the riser fibers from each customer site to the correct ports of the multiplexer/demultiplexer components. It is then necessary to verify that the appropriate connections have been made for each customer after fiber and multiplexer/demultiplexer installation. It may also be necessary to further evaluate these connections if a loss of light (LOL) or loss of signal (LOS) condition subsequently arises.
The use of optical time domain reflectometers (OTDRs) to diagnose optical fiber losses and faults is known in the art. An OTDR permits an operator to essentially “look into” an optical fiber and locate points of loss (e.g., splices, bends) and failure (e.g., breaks, separated connectors). An OTDR emits a pulse of light that is propagated through a fiber of interest. The OTDR time-gates the detected optical backscattered return that results from fiber inhomogeneities or sources of reflections or losses, to convert time delay to fiber position, thereby enabling the location of the loss/failure point to be ascertained. A conventional OTDR that operates on a single specified wavelength is impractical, however, for troubleshooting WDM systems because the multiplexer/demultiplexer equipment have ports that are opaque to light having wavelengths outside their respective passbands. This necessitates coupling the OTDR into the trunk fiber, leaving the mux, the fiber before the mux, the demux, and the fiber after the demux undiagnosable. This is disadvantageous, since some fraction of the known failures occur inside offices and at customer sites. Thus, it would be advantageous to include these optical components in the diagnostic procedures. OTDRs that operate on multiple-wavelengths are known in the art. For example, U.S. Pat. No. 6,445,445 to Nakayama et al. (“Nakayama”) discloses an OTDR that provides for switching the wavelengths of the OTDR's optical source. However, the Nakayama patent does not address diagnostics issues particular to WDM systems. The uses of multiple wavelength OTDRs in the current state of the art are primarily concerned with determining the amount of chromatic dispersion in a fiber link. This is determined by registering a feature on the OTDR trace (such as the end of the fiber) and, by justifying the time delays for the different wavelengths, determining the velocity of light at the different wavelengths and then fitting to a Sellmeier model for dispersion. In all such cases, the use of WDM systems is not anticipated because the properties of the Fabry-Perot lasers commonly used in these instruments are unsuitable for WDM transmission systems.
In other diagnostic schemes, as mentioned above, OTDRs at long wavelengths (such as 1625 nm, outside the conventional WDM bands) are used as an adjunct for trunk diagnostics. In these cases, a WDM filter is used to merge the OTDR wavelength onto and off of the trunk fiber. That is, the OTDR is used exclusively to check the integrity of the trunk, since the WDM filters prohibit that light from reaching either the mux or the demux. This method is not perfect for DWDM systems, but is commonly used because the optics for such systems is quite localized. Applications which we describe herein do not share this attribute of equipment localization, and for such applications the intra-trunk filters are not advantageous.